1. Field of the Invention
The present invention relates to an optical material containing a photochromic compound, and more particularly, it relates to a novel optical material having a high reflectance change, which is excellent in durability against repetitive coloring and decoloring.
2. Description of the Background Art
In recent years, study has been widely made on an optical recording medium employing a photochromic compound.
Upon irradiation with light of a prescribed wavelength, such a photochromic compound is changed in molecular structure by photochemical reaction to cause changes in optical properties such as absorbance, optical rotatory power, reflectiveness, a refractive index and the like. When the photochromic compound is irradiated width light of a specific wavelength, the molecular structure as changed returns to the original structure. Therefore, it is possible to record and reproduce information through such differences of the optical properties. Further, it is possible to erase the information by converting the molecular structure to the original structure.
For example, the Japanese journal "Bull. Chem. Soc. Jpn." (1990, Vol. 163, pp. 1311 to 1315) discloses 2,3-bis-(2-methylbenzo[b]thiophene-3-yl) maleic anhydride (which is a diarylethene photochromic compound) as this type of photochromic compound. This photochromic compound enters a photostationary state, i.e., a state containing molecules of both ring-opening and ring-closure states, to be colored red upon irradiation with light of a wavelength near 430 nm, for example, while the same enters a complete ring-opening state upon irradiation with light of a wavelength near 550 nm.
Therefore, it is possible to apply one of such reversibly changed states to a recorded state and the other one to an erased state. Further, it is possible to read information as recorded by irradiating the photochromic material with light of a specific wavelength (550 nm, for example) and detecting differences caused in optical properties such as absorbance between the two states. In general, the difference in absorbance, i.e., the difference in transmittance change or reflectance change between the two states is detected in a general reproducing method, and it is possible to obtain a superior signal as the difference in absorbance is increased. The difference in absorbance can be increased by the following methods:
(1) A method of increasing the concentration of the photochromic material contained in a thin film.
(2) A method of increasing the film thickness.
(3) A method of improving properties, such as a conversion rate and an absorption coefficient, of the photochromic material.
In relation to the aforementioned methods (1) to (3), it is known as to the method (1) that the conversion rate of the conventional diarylethene photochromic material is reduced as its concentration is increased, to have the maximum value in concentration of about 30 to 50 percent by weight with respect to a polymer and that no change in absorbance is improved even if the concentration is increased beyond this value.
As to the method (2), a laser beam which is narrowed through an objective lens is inevitably spread in the direction of depth if the thickness of a recording layer (photochromic material layer) is increased, and hence the recording density is reduced. Therefore, it is necessary to reduce the thickness of the recording layer at least to below 1 .mu.m, and hence the improvement of the absorbance change caused by increasing the film thickness is restricted.
The method (3) is adapted to improve the properties of the photochromic compound itself. The absorption coefficient (liter/mol.multidot.cm) means the light absorption ability of the photochromic compound, and the change in absorbance is increased as this value is increased. On the other hand, the conversion rate (%) is a value indicating the rate of molecules which are converted to ring-closure states from ring-opening states upon irradiation with light of a coloring wavelength up to a photostationary state. Thus, the change in absorbance is increased as the value of this conversion rate is increased.
In the conventional diarylethene photochromic compound, however, both of the absorption coefficient and the conversion rate are insufficient, and hence development of a photochromic compound having a large absorption coefficient and a high conversion rate is awaited.
It is also known that such a photochromic compound is deteriorated by a side reaction caused in a photoreaction process upon repetitive coloring and decoloring, and finally enters an unchanged state. The repeatable frequency for such coloring and decoloring corresponds to a reloadable frequency in a case of employing the photochromic compound as the material for an optical recording medium. Therefore, it is possible to improve reliability of the optical recording medium as durability against repetitive coloring and decoloring is improved. Thus, awaited is development of a material having excellent durability against repetitive coloring and decoloring, with improvement in absorption coefficient and conversion rate.